1. Field of the Invention
The present invention relates to a stepping motor control apparatus employing a control method for controlling the acceleration and deceleration of a stepping motor in accordance with a predetermined program.
2. Description of the Prior Art
Conventionally, a driving pulse train is set to have a frequency higher than a self-start frequency of a stepping motor so as to drive the stepping motor at a high speed in a manner to be described with reference to FIGS. 1 and 2.
FIG. 1 is a graph showing the torque of the stepping motor as a function of the driving frequency when a high-speed driving device is used. Referring to FIG. 1, dotted line L.sub.1 denotes a self-start frequency characteristic curve in the stepping motor; and L.sub.2, a continuous response characteristic curve.
FIG. 2 is a graph showing the driving frequency as a function of time elapsed (required) for acceleration. In acceleration control in FIG. 2, since the acceleration is kept constant, it is considered that the accelerating torque is also kept constant.
Referring again to FIG. 1, if a frequency lower than (plotted to the left of) a maximum continuous response frequency F.sub.2 plotted on the continuous response characteristic curve L.sub.2 is given as a target frequency f.sub.2, the accelerating torque must be smaller (e.g., Tb) than a motor output torque Ta at point P.sub.2 on the continuous response characteristic curve L.sub.2. The driving frequency is increased from a frequency lower than that corresponding to the stepping motor output torque Ta in the self-start frequency characteristic curve L.sub.1 to the target frequency f.sub.2 at a constant acceleration torque Tb.
However, as may be apparent from the curves L.sub.1 and L.sub.2 in the graph shown in FIG. 1, when the stepping motor is self-started, the output torque characteristic curve coincides with the curve L.sub.2, thereby forming a driving frequency interval wherein a torque is greater than the accelerating torque Tb by an amount Tc. As a result, it is found that the accelerating torque cannot be effectively utilized.
For this reason, it takes a long time to increase the driving frequency of the stepping motor to the target frequency, resulting in inconvenience. Furthermore, when gears are used in a power transmission system, noise due to back lash of the gears is caused by the excessive torque Tc.